Co-stimulatory molecules such as CD28 and CD40L are necessary for complete T cell activation and play a crucial role in mediating alloimmune responses and allograft rejection. Recently, members of the tumor necrosis factor receptor (TNFR) family have been implicated as additional co-stimulatory molecules that modulate alloimmune responses and graft rejection. CD30 is a pleiotropic TNFR family member that can promote cellular proliferation, cytokine production, and apoptosis depending on the cell type and its state of differentiation. However, little is known about the potential role of CD30+ T cells in alloimmune responses. Therefore, the proposed research plan is designed to test the hypothesis test that CD30 acts as a co-stimulatory molecule in allogeneic responses and modulates T cell proliferation and cytokine production and, subsequently, graft survival. (1) We will determine the effect of CD30 deficiency on T cell function in vivo and in vivo alloreactivity by (a) measuring proliferation, activation, and apoptosis of T cells in CD30-/- and CD30 ++ mice following in vivo and in vitro allostimulation using flow cytometry, CFSE staining, and PE-annexin V staining, (b) measuring T cell cytokine production in CD30-/- and CD30+/+ mice following in vivo and in vitro allostimulation using ELISPOT, ELISA, and RPA and (c) examining the effect of CD30 signal transduction on cell cycle and apoptosis related proteins by RPA and Western blot. (2) We will determine the effect of CD30 deficiency on allograft rejection and on the immune response to the graft by (a) measuring graft survival following vascularized heterotopic cardiac transplants in CD30 and CD30 mice and (b) measuring the function and the phenotype of graft infiltrating cells following allograft transplantation in CD30"/"and CD30 +/+mice using ELISA, cellular proliferation assays, cytotoxicity assays, flow cytometry, H & E staining, and immunohistochemistry. Understanding the contribution of CD30 to alloreactivity will provide novel opportunities to modulate the function of allospecific T lymphocyte populations.